Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityNeural codesPSY 310Greg FrancisLecture 11Who is this guy?Purdue UniversityDorsal stream From visual cortex to the parietal lobe Involved in working with objectsPurdue UniversityVentral stream From visual cortex to the temporal lobe Involved in recognizing or identifying objectsPurdue UniversityCortex structure Many differentdefined “areas”of cortex Dorsal stream Lots ofdifferent areasmake up thisstream Different areasare specializedfor certainproperties ofthe stimulus E.g MTPurdue UniversityReceptive fields Vary not just in space, but in time Cells respond best to stimuli moving in certaindirections What kind of stimuli should you use? What produces “pure motion”? Random dots Horse2.movPurdue UniversityReceptive fields Vary the direction of the dotsProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityReceptive fields Vary the consistency of the dots Iseqa.movPurdue UniversityMT cell responses Movie MT-direction.movTimePreferred directionNull directionPurdue UniversityMT cell responses Movie MT-strength.movTime99% correlated26% correlatedPurdue UniversityVentral stream We’ll talk more about motion later in the course Now we turn to the ventral stream of processingPurdue UniversityInferotemporal Cortex Cells in IT respond to lots of different stimuli How do you deduce what really matters? This cell seems to respond to the cross (+) pattern This includes many different stimuliPurdue UniversityIdentifying receptive fields Start withlots ofdifferentstimuli Show eachoneseparately Recordresponsefrom a cellProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityIdentifying receptive fields Start with lotsof differentstimuli Show eachoneseparately Recordresponsefrom a cell This oneresponds tothe chairPurdue UniversityIdentifying receptive fields Start with lotsof differentstimuli Show eachoneseparately Recordresponsefrom anothercellPurdue UniversityIdentifying receptive fields Start with lotsof differentstimuli Show eachoneseparately Recordresponsefrom anothercell This oneresponds tolots of stimuliPurdue UniversityReceptive field How to figure what part of the stimulus really “drives”the cell? Find a stimulus that makes lots of action potentials Top view of a stuffed tiger dollPurdue UniversityReceptive field Make simplified versions ofthe stimulus For this particular neuron, allof these stimuli generate astrong responsePurdue UniversityReceptive field Find a stimulus that makeslots of action potentials Top view of a stuffed tigerdoll Cell response drops withfurther simplificationsProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityCritical features So the cell responds strongly to the tiger doll head Because that stimulus contains features that arepresent in the simplified versionPurdue UniversityCritical features Do this over and over again for different cells Starting imagePurdue UniversityCritical features Do this over and over again for different cells Simplified image with critical featuresPurdue UniversityFaces People are very good at recognizing faces Brain damage can lead to specific problemsrecognizing faces We see faces where they do not existPurdue UniversityFace perception Our percepts of faces are so strong that they over-come other kinds of information Watch this video of a mask Chaplin.movPurdue UniversityFace perception What does the brain respond to in a face? We can study both human behavior and neuralbehavior Face adaptationProf. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityFace perception What does the brain respond to in a face? We can study both human behavior and neural behavior Face adaptation Stare at this face for 10-20 secondsPurdue UniversityFace perception What does the brain respond to in a face? We can study both human behavior and neural behavior Face adaptation Who is this?Purdue UniversityFace perception What does the brain respond to in a face? We can study both human behavior and neural behavior Face adaptation Are you sure?Purdue UniversityFace adaptation It’s the same idea as for spatial frequencyadaptation Cells responding to features in Bush’s face“fatigue” and do not respond as strongly to theambiguous face Cells responding to features in Kerry’s face arenot “fatigued” and so respond normally The stronger responses from the Kerry-featuresmakes the face look like Kerry Exactly what those features are is complicatedPurdue UniversityConclusions Dorsal stream Motion perception Ventral stream Object recognition Faces Getting neural data is quite difficult!Purdue UniversityNext time Distributed coding How cells develop complex receptive fields Attention and
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